In gas turbines, as well as certain other machines having moving parts, increased efficiency may be achieved by minimizing and maintaining clearance between a rotating part and the surrounding structure, such as minimizing the clearance between rotating buckets and a surrounding shroud. Various sealing structures, such as honeycomb structures having repeating units of open hexagonal prisms are known, but include limitations.
Known honeycomb sealing structures are not formed from hard-to-weld (HTW) alloys, such as nickel-based superalloys and certain aluminum-titanium alloys, due to their gamma prime and various geometric constraints, which are susceptible to gamma prime strain aging, liquation and hot cracking. These HTW alloys are also difficult to join when the gamma prime phase is present in volume fractions greater than about 30%, which may occur when aluminum or titanium content exceeds about 3%. Further, honeycomb sealing structures may require significant oxidation resistance, and nickel-based superalloys which have significant oxidation resistance typically do not exhibit acceptable weldability for commercial processes. As used herein, an “HTW alloy” is an alloy which exhibits liquation, hot and strain-age cracking, and which is therefore impractical to weld.
Additionally, known honeycomb sealing structures formed by known methods include homogenous patterns of the honeycomb structure, are oriented such that the open hexagonal prism repeating units of the honeycomb structure extend from the surface on which they are disposed at an orthogonal angle, and are limited to open hexagonal prism repeating units. As used herein, “open” in relation to polyhedrons such as prisms refers to a polyhedron which is missing a face, such as, in the case of a honeycomb structure, missing the open base face distal from the surface on which the sealing structure rests, thereby presenting open hexagonal cells.